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ON – OFF controllers

Proportional Controllers

Proportional Derivative Controllers

Proportional Integral Controllers

Proportional Integral Derivative Controllers

Ratio Controllers

Adaptive Controllers

Programmable Logic Controllers

Fuzzy Logic Controllers

Distributed Control Systems

SCADA

Intelligent Controllers

Computer Based controllers

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The ratio control architecture is used to maintain the flow rate of one stream in a process at a defined or specified proportion relative to that of another. A common application for ratio control is to combine or blend two feed streams to produce a mixed flow with a desired composition or physical property

Ratio Controllers

An important control problem in the food processing industry is the combining of two or more streams to provide a mixture having a desired ratio of components. Examples of this mixing operation include the blending of oils/fats/essences/juices entering a reactor or for the injection of a fuel/air mixture into a furnace.

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In a flow control loop, the dynamic elements consist of the controller, the flow-measuring element, and the control valve.
For incompressible fluids, there is no lag between the change in valve position and the corresponding flow rate. For this reason, the transfer function between the valve and the measurement of flow rate is simply unity.
The block diagram also shows a transfer function Gp that relates the flow rate of B to the supply pressure of B.
A transfer function Gm1 is also shown that represents the dynamic lag of the flow measuring element for stream A.

From the block diagram, the flow of B may be written

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The control action for a flow control system is usually PI.
The integral action is needed to eliminate offset and thereby establish a precise ratio of the mixed streams of A and B.
Derivative action is usually avoided in flow control because the signal from a flow measuring element is inherently noisy.
The presence of derivative action would amplify the noise and give poor control.

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Relay is typically one of two types:� A ratio relay, where the mix ratio is entered once during configuration and is generally not available to operations staff during normal operation.� A multiplying relay (shown), where the mix ratio is presented as an adjustable parameter on the operations display and is thus more readily accessible for change.

The relay multiplies the measured flow rate of the wild feed stream, PVw, by the entered mix ratio to arrive at a desired or set point value, SPc, for the controlled feed stream. A flow controller then regulates the controlled feed flow rate to this SPc, resulting in a mixed flow stream of specified proportions between the controlled and wild streams.

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Adaptive control is the control method used by a controller which must adapt to a controlled system with parameters which vary, or are initially uncertain.

For example, as an aircraft flies, its mass will slowly decrease as a result of fuel consumption; a control law is needed that adapts itself to such changing conditions. Adaptive control is different from robust control in that it does not need a prior information about the bounds on these uncertain or time-varying parameters; robust control guarantees that if the changes are within given bounds the control law need not be changed, while adaptive control is concerned with control law changing itself.

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Applications

Self-tuning of subsequently fixed linear controllers during the implementation phase for one operating point;
Self-tuning of subsequently fixed robust controllers during the implementation phase for whole range of operating points;
Self-tuning of fixed controllers on request if the process behaviour changes due to ageing, drift, wear, etc.;
Adaptive control of linear controllers for nonlinear or time-varying processes;
Adaptive control or self-tuning control of nonlinear controllers for nonlinear processes;
Adaptive control or self-tuning control of multivariable controllers for multivariable processes (MIMO systems);

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